If this is your only reason for wanting to test the employee, then no, you should not test this employee because of reasonable suspicion of drug use. The so-called known drug user may not be using drugs, and just because an employee is talking to this person does not mean that the employee is taking drugs.
In states where drug testing is not restricted, nothing prohibits a drug test. However, in states that do not permit suspicion-less drug testing (such as random tests), this would not be allowed.